Field of the Invention
The present invention relates to an image pickup apparatus, such as a digital still camera, a control method thereof, and a storage medium storing a control program therefor, and in particular, relates to a method for detecting flicker and reducing its effect when taking a picture under an artificial lighting.
Description of the Related Art
A recent image pickup apparatus, such as a digital still camera, performs an automatic exposure decision process (AE), an automatic white balance process (AWB), and a characteristic-color extracting process, and object recognition processes, such as a tracking process and a face recognition process.
Furthermore, since a sensitivity of an image pickup device of an image pickup apparatus becomes higher, an image without blur can be taken with a high-speed shutter even under a comparatively dark environment like an interior of a room. When a picture is taken in a room under fluorescent light that is spread as an indoor light source, a flicker phenomenon in which illumination light swings periodically at power supply frequency occurs.
The high-speed shutter photographing under such a flickering light source may vary a color temperature for every frame due to the effect of flicker. Furthermore, exposure unevenness and color unevenness may occur in one frame.
In order to reduce the effect of flicker, there is a known image pickup apparatus that detects flicker and controls exposure according to the detection result, for example (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-193922 (JP 2004-193922A)). This image pickup apparatus reduces the effect of flicker by exposing at the peak position of the flicker where the variation of brightness is minimized according to the detection result of the flicker.
Incidentally, the above-mentioned publication does not disclose a technique for detecting flicker. There is a technique that detects a flicker cycle in response to transition of a photometry value for every charge storage by continuously storing with an image pickup device of which the charge storage cycle is different from an assumed flicker cycle.
However, if the flicker cycle is detected by the above-mentioned technique, charge has to be stored continuously. If an object recognition process and the charge storage process other than the detection of the flicker cycle are performed at the time of photographing, for example, processing load increases and processing time gets longer. Accordingly, a continuous photographing frame speed drops in a case where a user continuously takes pictures with the image pickup apparatus.